Bipolar junction transistors (BJT) and heterojunction bipolar transistor (HBT) integrated circuits (ICs) have developed into an important technology for a variety of applications, particularly as power amplifiers for wireless handsets, microwave instrumentation, and high speed ( greater than 10 Gbit/s) circuits for fiber optic communication systems. Future needs are expected to require devices with lower voltage operation, higher frequency performance, higher power added efficiency, and lower cost production. The turn-on voltage of a BJT or HBT is defined as the base-emitter voltage (Vbe) required to achieve a certain fixed collector current density (Jc). The turn-on voltage can limit the usefulness of devices for low power applications in which supply voltages are constrained by battery technology and the power requirements of other components.
Unlike BJTs in which the emitter, base and collector are fabricated from one semiconductor material, HBTs are fabricated from two dissimilar semiconductor materials in which the emitter semiconductor material has a wider band gap than the semiconductor material from which the base is fabricated. This results in a superior injection efficiency of carriers from the base to collector over BJTs because there is a built in barrier impeding carrier injection from the base back to the emitter. Selecting a base with a smaller band gap decreases the turn-on voltage because an increase in the injection efficiency of carriers from the base into the collector increases the collector current density at a given base-emitter voltage.
HBTs, however, can suffer from the disadvantage of having an abrupt discontinuity in the band alignment of the semiconductor material at the heterojunction can lead to a conduction band spike at the emitter-base interface of the HBT. The effect of this conduction band spike is to block electron transport out of the base into the collector. Thus, electron stay in the base longer resulting in an increased level of recombination and a reduction of collector current gain. Since, as discussed above, the turn-on voltage of heterojunction bipolar transistors is defined as the base-emitter voltage required to achieve a certain fixed collector current density, reducing the collector current gain effectively raise the turn-on voltage of the HBT. Consequently, further improvements in the fabrication of semiconductor materials of HBTs are necessary to lower the turn-on voltage, and thereby improve low voltage operation devices.
The present invention provides an HBT having an n-doped collector, a base formed over the collector and composed of a III-V material that includes indium and nitrogen, and an n-doped emitter formed over the base. The III-V material of the base layer has a carbon dopant concentration of about 1.5xc3x971019 cmxe2x88x923 to about 7.0xc3x971019 cmxe2x88x923. In a preferred embodiment, the base layer includes the elements gallium, indium, arsenic, and nitrogen. The presence of nitrogen in the material and the high dopant concentration of the materials of the invention reduce the band gap and the sheet resistivity (Rsb)of the material which results in a lower turn-on voltage. The HBTs of the present invention have a lower turn-on voltage than GaAs-based HBTs of the prior art.
In a preferred embodiment, the III-V compound material system can be represented by the formula Ga1-xInxAs1-yNy. It is known that the energy-gap of Ga1-xInxAs drops substantially when a small amount of nitrogen is incorporated into the material. Moreover, because nitrogen pushes the lattice constant in the opposite direction from indium, Ga1-xInxAs1-yNy, alloys can be grown lattice-matched to GaAs by adding the appropriate ratio of indium to nitrogen to the material. Thus, excess strain which results in an increased band gap and misfit dislocation of the material can be eliminated. The ratio of indium to nitrogen is thus selected to reduce or eliminate strain. In a preferred embodiment of the present invention, x=3y in the Ga1-xInxAs1-yNy base layer of the HBT.
In one embodiment, the transistor is a double heterojunction bipolar transistor (DHBT) having a base composed of a semiconductor material which is different from the semiconductor material from which the emitter and collector are fabricated. In a preferred embodiment of a DHBT, the Ga1-xInxAs1-yNy base layer can be represented by the formula Ga1-xInxAs1-yNy, the collector is GaAs and the emitter is selected from InGaP, AlInGaP and AlGaAs.
Another preferred embodiment of the invention relates to a HBT or DHBT in which the height of the conduction band spike is lowered in combination with lowering of the base layer energy gap (Egb). Conduction band spikes are caused by a discontinuity in the conduction band at the base/emitter heterojunction or the base/collector heterojunction. Reducing the lattice strain by lattice matching the base layer to the emitter and/or the collector layer reduces the conduction band spike. This is typically done by controlling the concentration of the nitrogen and the induim in the base layer. Preferably, the base layer has the formula Ga1-xInxAs1-yNy wherein x is about equal to 3y.
In one embodiment, the base can be compositionally graded to produce a graded band gap layer having a narrow band gap at the collector and a wider band gap at the emitter. For example, a Ga1-xInxAs1-yNy base layer of a DHBT can be graded such that x and 3y are about equal to 0.01 at the collector and are graded to about zero at the emitter. The base layer can also be dopant graded such that the dopant concentration is higher near the collector and decrease gradually across the thickness of the base to the base emitter heterojunction. Methods of forming graded base layers are known to those skilled in the art and can be found on pages 303-328 of Ferry, et al., Gallium Arsenide Technology (1985), Howard W. Sams and Co., Inc. Indianapolis, Ind., the entire teachings of which are incorporated herein by reference.
Another method of minimizing the conduction band spike is to include one or more transitional layer between the heterojunction. Transitional layers having low band gap set back layers, graded band gap layers, doping spikes or a combination of thereof can be used to minimize the conduction band spike. In addition, one or more lattice-matched layers can be present between the base and emitter or base and collector to reduce the lattice strain on the materials at the heterojunction.
The present invention also provides a method of fabricating an HBT and a DHBT. The method involves growing a base layer composed of gallium, indium, arsenic and nitrogen over an n-doped GaAs collector. The base layer is grown using an internal and external carbon source to provide carbon doped base layer. An n-doped emitter layer is then grown over the base layer. The use of an internal and external carbon source to provide the carbon dopant for the base layer results in a material with a higher carbon dopant concentration than has been achieved in the prior art. Typically, dopant levels of about 1.5xc3x971019 cmxe2x88x923 to about 7.0xc3x971019 cmxe2x88x923 are achieved using the method of the invention. In a preferred embodiment, dopant levels of about 3.0xc3x971019 cmxe2x88x923 to about 7.0xc3x971019 cmxe2x88x923 are achieved with the method of the invention. A higher dopant concentration in a material reduces the sheet resistivity and band gap of the material. Thus, the higher the dopant concentration in the base layer of an HBT and DHBT, the lower the turn on voltage of the device.
The present invention also provides a material represented by the formula Ga1-xInxAs1-yNy in which x and y are each, independently, about 1.0xc3x9710xe2x88x924 to about 2.0xc3x9710xe2x88x921. Preferably, x is about equal to 3y. More preferably, x and 3y are about equal to 0.01. The material is doped with carbon at a concentration of about 1.5xc3x971019 cmxe2x88x923 to about 7.0xc3x971019 cmxe2x88x923. Preferably, the carbon dopant concentration is about 3.0xc3x971019 cmxe2x88x923 to about 7.0xc3x971019 cmxe2x88x923.
The reduction in turn-on voltage allows for better management of the voltage budget on both wired and wireless GaAs-based RF circuits, which are constrained either by standard fixed voltage supplies or by battery output. Lowering the turn-on voltage also alters the relative magnitude of the various base current components in a GaAs-based HBT. DC current gain stability as a function of both junction temperature and applied stress has been previously shown to rely critically on the relative magnitudes of the base current components. The reduction in reverse hole injection enabled by a low turn-on voltage is favorable for both the temperature stability and long-term reliability of the device. Thus, strain free Ga1-xInxAs1-yNy base materials having a high dopant concentration enhance RF performance in GaAs-based HBTs and DHBTs.